Yeah you're absolutely right but it really ties it all together. I think this is the best explanation of why you see things in older Dooms in Doom 2016 and vice versa. I'll take this as truth since it's better then any other theory I've seen. Plus it makes the lore so much more awesome.

This theory doesn't explain why if the whole thing started in the 1st age & the game happens in the 4th age, how/why would Mars now be an alternate dimension? Never explains how Doom 3 ties in as D3 has easter eggs too & they're the same ones used in Doom 2016. Also ignores the original Doom level secrets. Also ignores that in Doom 2016 Demons & Hell wearn't the aggressors, says UAC invaded them (D3 suggest this too until you find out Betruger is a demon and that was the plan to let Hell in)

While he points out that in the end of Episode 1 of Doom you do die & go to hell, you prevail in the end (Ep 3) & he assumes, with nothing but Doom 2016 existing as proof, the Doom marine in Episode 2, 3, 4 & Doom 2 are now possessed. The text in those games doesn't suggest that at all. His theory says that Doom 1/2 didn't end like they did. Doom 2 even says you go back to Earth & help rebuild.

The alternate universe idea works but unfortunately that works in nearly any circumstance so I don't count that unless id/beth specifically says so.

My theory would be this (I like this one for the Zelda universe even though it isn't official & Nintendo has stated that it's a multiuniverse):

Doom 1/2 happened as said in ending texts. Eventually most humanity left the solar system (always stragglers behind). They had the ships & tech to do it eventually. Things left behind would reveal to future generations what happened.

Doom 3: The human stragglers eventually forget what happened & start their own new society. Demons have a bigger influence in (earth) humanity this time and try to recreate the failed invasion from Doom 1/2. Hence you find evidence of the last invasion on Mars. It would be assumed there's evidence on Earth but since people lived there odds are it would be destroyed in the name of progress (well, except the pyramids. Those ship loading/launching platforms are still there. ) Really, if people did find evidence of hell's invasion & records of what's going on, the majority of people would assume it "ancient" civilization superstition. Anyway, someone saves the day again. As evidenced by RoE Hell doesn't end. Worth noting D3 & RoE doesn't say how things turned out eventually like D1/2 endings did.

Doom 2016: Hell's having management issues from all the issues the people are causing. The doom slayer is the guy from Doom 1/2 except that it's a "stripped" down version of him. He did leave hell but part stayed behind somewhat (like a carbon copy). Doom 2016 goes in the the depth of the history of how Hell became what it is by Doom 2016's point. I'd say the 2nd age is ~time of Doom 1/2 & 3rd age is D3. First age would be before this. I agree with the guy that time moves different in Hell. I'd say ours = faster, so to us thousands of years = hundreds in Hell. As for the time period of Doom 2016 vs D3, I'd say at one time humans reset the calendar (again). Perhaps based on the incidents in D3, that was eventually reset to 0 (that would mean D2016 takes place ~2000 years after D3, the time span between D1 & 2016 would be ~4000-6000 years). Humans in Earth area advanced quite a bit. Some people believe that the stuff found in D3 is significant & it turns in to a religion. UAC finally has tech to "handle" Hell (kind of like how a submarine can handle the water as long as no holes in the hull) but gets infiltrated by those who worship the demons (most likely converted after working at the company. The religious beliefs = great work ethic for current UAC project) & what not & it's released again. As you play you find out your demonic history.

Doom 1 secret levels are an attempt by Hell for f*ck with Doom Slayer's head.

That's my theory as to why we see Doom 1/2/3 evidence in the game. Final Doom has evidence of we're past Mars & have better tech (on a moon of Jupiter & devices that can override hell's teleporters & shut them down). Final Doom also gives us a big spaceship that can be reverse engineered for further space colonization. Icon of Sin is also in Final Doom, which means there's more then one. We don't know which one is in Doom 2016.